VISUALLY GUIDED REACHING WITH THE FORELIMB CONTRALATERAL TO A BLIND HEMISPHERE - A METABOLIC MAPPING STUDY IN MONKEYS

The 2-C-14-deoxyglucose method was used to map local cerebral metaboli c activity in monkeys performing a unimanual task requiring visually g uided arm reaching and key pressing. The study was carried out with mo nkeys that either had intact brains or had one hemisphere deprived of visual input by unilateral optic tract section combined in some cases with forebrain commissurotomy. The metabolic mapping revealed activati on of sensorimotor cortex only in the hemisphere contralateral to the moving forelimb, irrespective of whether this hemisphere was intact or visually deafferented. These results suggest that visually guided rea ching with the forelimb contralateral to the ''blind'' hemisphere is s ub-served by that hemisphere's sensorimotor cortex and not by the cort ex of the ipsilateral, ''seeing'' hemisphere. Other areas that were mo re active metabolically in the ''blind'' than in the ''seeing'' hemisp here included the supplementary motor, the secondary somatosensory, an d certain posterior parietal cortical areas, intraparietal lateral 5 ( lateral 5-ip), 7a, and intraparietal 7 (7-ip). It is suggested that th e ''blind'' hemisphere utilizes at least two distinct pieces of inform ation to guide forelimb movements to visual targets: (1) information a bout the location of the visual target derived from head and eye movem ents made to this target and mediated via the inferior parietal cortic al areas 7a and 7-ip, and (2) information about the instantaneous uppe r extremity position derived from forelimb proprioceptive mechanisms a nd mediated via the somatosensory cortex and thereafter via the superi or parietal cortical area, lateral 5-ip.